95 mitsubishi tranny problem

I just purchased a 95 mitsubishi pick up. I think it is a mighty max. It has a 5 speed manuel transmission. It won't stay in 2nd or 4th gear. If the truck is moving then you cannot down shift at all. Is this an expensive repair or is it a common tranny that maybe I can find in other types of vehicle. Any info would be greatly appeciated. Thanks Ty
One more thing the clutch seems to be working just fine and the tranny works great in 1st 3rd and 5th gears.

I saw one on e-bay that would fit for $105

What kind of luck did you have with your tranny problem? Could you fix it???

i had to take my seats out pull my carpet up and get to the shift knob, I checked if the knob is properlly in the joint if that doesnt work, you need a new clutch even if it doesnt feel like it, my truck is doing the same thing. the guy at a autoshop said my PRESSURE PLATE is going out and it would cost about $400 to fix possibly even $500

took truck to a tranny shop, found out a rolled pin that holds a gear in place had come loose. The pin was in the main trans shaft. To fix was major $$$. approx $700. Got another tranny from junkyard out of a 93 dodge d-50. Now works great. Note... alot of parts from the D-50 will fit the mighty max..Good luck.